[docs] add zh docs (#3507)

* [docs] add zh docs

* [docs] add lang dropdown

* [docs] update mkdocs zh config

* [docs] migrate assets

* [docs] update overrides dir in mkdocs zh config

* [docs] exclude locales director in main mkdocs config

* [docs] rename assets to public to avoid conflicting with template

* [docs] extra_css change followup

* [docs] add theme.palette.toggle.icon back into mkdocs zh config

* [docs] fix zh readme reference + migrate language-specific repo markdown to docs

* [docs] translate remaining repo docs + update reference

* [docs] update zh index.md reference

* [docs/zh] wording alignment

docs/user_guide/custom_css.md

@@ -14,7 +14,7 @@ This allows you to customize the appearance of your GoToSocial profile for users
 
 Here's a standard GoToSocial profile page:
 
-![A GoToSocial test profile page. The standard color scheme of grey, blue, and orange.](./../assets/cssstandard.png)
+![A GoToSocial test profile page. The standard color scheme of grey, blue, and orange.](./../public/cssstandard.png)
 
 Let's say we want the background color to be black instead of grey.
 
@@ -30,7 +30,7 @@ We then click on Save Profile Info.
 
 If we go back to our profile page and refresh the page, it now looks like this:
 
-![The same GoToSocial test profile page. The background is now black.](./../assets/cssblack.png)
+![The same GoToSocial test profile page. The background is now black.](./../public/cssblack.png)
 
 If we want to get really fancy, we can add an ombre effect to the background, by using the following CSS code instead:
 
@@ -42,7 +42,7 @@ If we want to get really fancy, we can add an ombre effect to the background, by
 
 After saving the css and refreshing the profile page, the profile now looks like this:
 
-![The same GoToSocial test profile page. The background now starts dark red and fades to purple further down the page.](./../assets/cssgradient.png)
+![The same GoToSocial test profile page. The background now starts dark red and fades to purple further down the page.](./../public/cssgradient.png)
 
 ## Accessibility